The first thing to realize about installing fiberglass insulation is that you can’t just shove the batts into wall and ceiling cavities any old way and expect them to do their job. Insulation batts must fit snugly between studs or joists and cannot be jammed in tightly or packed loosely and sloppily with gaps all over the place. Kraft paper-faced batts have tabs, which should be stapled to the face of the studs every 12 in. or so.
Many installers staple the tabs to the insides of the studs, which makes it easier to install drywall. However, I don’t recommend this approach. When the paper tabs are stapled to the inside of the studs, a slight gap is left along both sides. Gaps are absolutely taboo when you’re installing fiberglass insulation because they significantly reduce the insulating value.
If you plan to install fiberglass insulation in warm, humid climates (such as the southeastern U. S.), buy unfaced batts (see the photo at right). Fiberglass insulation with kraft paper-facing acts as a vapor barrier on the inside of exterior walls, potentially causing moisture problems.
For partial bays (less than 14/ in. or 22/ in. wide between studs) and small spaces, such as over headers and under windows, insulation must be cut to fit. Measure the width and length of smaller bays and cut the insulation about к in. to 1 in. larger (no more!) in each direction so that it will fit snugly in the cavities (see the sidebar on p. 202). You don’t need to staple smaller pieces of insulation in place; the snug fit should hold them until the dry — wall is installed.
While you’re insulating, keep in mind that another property of insulation is sound suppression. Given its relatively reasonable cost, you may want to use unfaced insulation in bedroom walls that adjoin a bathroom, living room, or utility room.
Take even more care when insulating the ceiling (see the top photo on p. 203). Any heat that escapes into the attic can cause snow to melt, possibly causing an ice dam on your roof. When insulation batts butt together end to end in the ceiling, make sure the joints are tight.
Because of the importance of keeping heat in the living area and out of the attic, I prefer using blown-in cellulose for the attic, even if the walls are insulated with fiberglass batts. Cellulose settles into and around gaps in the framing, forming what amounts to a giant down comforter over the entire living area of your house. And remember, it doesn’t cost much to add a few more inches of cellulose—
